Method for forming semiconductor film and use of semiconductor film

ABSTRACT

The present invention provides a process for forming a semiconductor film, comprising the steps of applying a semiconductor particle dispersion liquid to a substrate surface by spray coating in such a manner that the atomized droplets of the dispersion liquid discharged from the spray coater have a mean diameter of about 30 μm or less, and drying the coating to form a porous semiconductor film; and use of the semiconductor film obtained by the process.

TECHNICAL FIELD

The present invention relates to a process for forming a semiconductorfilm and use of the semiconductor film.

BACKGROUND ART

Semiconductors such as titanium oxide, due to their photocatalyticactivity, have antibacterial properties, hydrophilicity, stainresistance, anti-fogging properties, gas decomposing properties,deodorizing properties, water-treating capabilities, energy conversionproperties and other properties, and thus are used in a variety offields.

In particular, photoelectric conversion devices, such as solar cells,that utilize the energy conversion properties of titanium oxide andother semiconductors are attracting attention as a means for producingelectrical energy without adverse effects on the global environment.

When using titanium oxide or other semiconductors as photocatalysts, itis preferable to form them into porous films on substrates.

With regard to processes for forming a porous film of semiconductor,such as titanium oxide, Japanese Unexamined Patent Publication No.1998-212120 discloses a process for forming a porous titanium oxidefilm, comprising applying a dispersion of titanium oxide particles in aglyme solvent, onto a glass, metal, ceramic or like substrate by spraycoating, dip coating or like process, and then baking the coating at 200to 800° C.

Further, Japanese Unexamined Patent Publication No. 2002-145615discloses a process for forming a porous titanium oxide film on asubstrate, comprising intermittently spraying a starting solutionobtained by adding hydrogen peroxide or aluminum acetylacetonate to atitanium oxide precursor, onto a substrate such as glass maintained at ahigh temperature of 350° C. or 500° C. so as to thermally decompose thetitanium oxide precursor to titanium oxide.

However, these processes have a drawback in that they involve heating ata high temperature of 200° C. or more to form a porous titanium oxidefilm, and therefore are not applicable to thermoplastic resin substratesthat deform or degrade at temperatures of 200° C. or higher.

Japanese Unexamined Patent Publication No. 1999-204152 discloses aprocess comprising applying a dispersion of metal oxide particles in ahigh polymer material solution to a high polymer film provided with aconductive layer, and drying the dispersion at a temperature of 200° C.or lower.

This process employs a relatively low heating temperature, and thus iscapable of forming a titanium oxide film on thermoplastic resinsubstrates. In this process, however, since the metal oxide particlesare dispersed in a high polymer material solution, the obtained titaniumoxide film has good adhesion but is unlikely to be porous. Thus, theprocess has a drawback in that the resulting film has inferior energyconversion properties, such as photoelectric conversion efficiency.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a process for forming asemiconductor film that has excellent adhesion to substrates, includingthermoplastic resin substrates such as high polymer films, and that hasexcellent photocatalytic properties such as energy conversion activity.

The present inventors conducted extensive research to achieve the aboveobject. As a result, the inventors found that a preferable poroussemiconductor film can be formed by applying a dispersion liquid ofsemiconductor particles to a substrate by spray coating whilecontrolling the atomized droplets of the dispersion liquid dischargedfrom the spray coater to have a mean diameter of 30 μm or less, therebyachieving the above object.

The present invention was accomplished by further studies based on theabove novel finding.

The present invention provides the following processes for forming asemiconductor film, and use of the semiconductor films formed by theseprocesses.

1. A process for forming a semiconductor film, comprising the steps of:

applying a semiconductor particle dispersion liquid to a substratesurface by spray coating in such a manner that the atomized droplets ofthe dispersion liquid discharged from the spray coater have a meandiameter of about 30 μm or less; and

drying the coating to form a porous semiconductor film.

2. The process according to item 1, wherein the substrate is athermoplastic resin substrate.

3. The process according to item 2, wherein the thermoplastic resinsubstrate is a high polymer film.

4. The process according to item 1, wherein the semiconductor particledispersion liquid is a dispersion in methanol and/or ethanol ofparticles of at least one semiconductor selected from the groupconsisting of metal oxides, perovskites, metal sulfides and metalchalcogenides.

5. The process according to item 4, wherein the semiconductor particlesare titanium oxide particles.

6. The process according to item 5, wherein the titanium oxide particlesare anatase-type titanium oxide particles.

7. The process according to item 1, wherein the semiconductor particledispersion liquid has a solids content of about 1 wt. % to about 40 wt.%.

8. The process according to item 1, wherein the semiconductor particledispersion liquid has a viscosity of about 0.001 Pa·sec to about 2Pa·sec.

9. The process according to item 1, wherein the atomized droplets of thedispersion liquid discharged from the spray coater have a mean diameterof about 1 μm to about 25 μm.

10. The process according to item 1, wherein the coating is dried byheating at a temperature of about 200° C. or less or by irradiation withelectromagnetic waves.

11. The process according to item 10, wherein the electromagnetic wavesare microwaves.

12. A photocatalyst comprising a porous semiconductor film formed on asubstrate by the process according to item 1.

13. The photocatalyst according to item 12, wherein the poroussemiconductor film is a porous titanium oxide film.

14. The photocatalyst according to item 13, wherein the porous titaniumoxide film is a porous anatase-type titanium oxide film.

15. A photoelectrode for dye-sensitized solar cells, comprising a poroussemiconductor film formed by the process according to item 1 on anelectrically conductive transparent layer formed either on a glass plateor a transparent high polymer film.

16. A photoelectrode according to item 15, wherein the poroussemiconductor film is a porous titanium oxide film.

17. A photoelectrode according to item 16, wherein the porous titaniumoxide film is a porous anatase-type titanium oxide film.

Process for Forming Semiconductor Film

The process for forming a semiconductor film according to the presentinvention comprises the steps of applying a semiconductor particledispersion liquid to a substrate surface by spray coating in such amanner that the atomized droplets of the dispersion liquid dischargedfrom the spray coater have a mean diameter of about 30 μm or less, andthen drying the coating to form a porous semiconductor film.

Substrate

The substrate for use in the process of the present invention is notlimited and may be selected from various known substrates. Specifically,usable substrates include sheets, films, molded articles and othersubstrates made of thermoplastic resins; glass, metals, concrete andother inorganic substrates; etc. These substrates may be previouslyprovided with primer coatings, electrically conductive layers or thelike.

Preferable examples of glass, an inorganic substrate, include soda-limeglass, which has cost and strength advantages; and no-alkali glass,which is not degraded by alkali elution.

Preferable thermoplastic resin substrates are high polymer films.Preferable materials for high polymer films include polyethyleneterephthalate, triacetyl cellulose, polyethylene naphthalate,syndiotactic polystyrene, polyphenylene sulfide, polycarbonates,polyallylates, polysulfones, polyester sulfones, polyimides, polyetherimides, cyclic polyolefins, phenoxy bromide resins, silicon resins,fluororesins, acrylic resins, etc. From the viewpoint of practical use,polyethylene terephthalate films are particularly preferable as highpolymer films.

The high polymer films preferably have high flexural strength and hightransparency.

The high polymer films are not limited in shape and may be rectangular,square or other shapes. For instance, rolled high polymer film stripsare usable. When a large-sized high polymer film or a high polymer filmstrip is used, the high polymer film can be cut to a desired size afterbeing coated with the semiconductor particle dispersion liquid anddried.

With respect to the size of the polymer film, for example when arectangular or square film is used, it is about 1 cm to about 10 m,preferably about 5 cm to about 5 m, and more preferably about 10 cm to 2m, in terms of both length and width. When a film strip is used, itswidth is about 1 cm to about 10 m, preferably about 5 cm to about 5 m,and more preferably about 10 cm to about 2 m. The strip is used asrolled, and therefore, the length of the film strip is not limited. Thehigh polymer film is preferably about 1 μm to about 10 mm thick, andmore preferably about 5 μm to about 5 mm thick.

Semiconductor Particle Dispersion Liquid

The semiconductor particle dispersion liquid for use in the process ofthe present invention is obtained by dispersing semiconductor particlesin a solvent.

The semiconductor particles are not limited and may be any knownsemiconductor particles. Usable semiconductors include titanium oxide,zinc oxide, manganese oxide, cadmium oxide, indium oxide, lead oxide,molybdenum oxide, tungsten oxide, antimony oxide, bismuth oxide, copperoxide, mercury oxide, silver oxide, manganese oxide, iron oxide,vanadium oxide, tin oxide, zirconium oxide, strontium oxide, galliumoxide, silicon oxide, chromium oxide and other metal oxides; SrTiO₃,CaTiO₃ and other perovskites; cadmium sulfide, zinc sulfide, indiumsulfide, lead sulfide, molybdenum sulfide, tungsten sulfide, antimonysulfide, bismuth sulfide, cadmium zinc sulfide, copper sulfide and othermetal sulfides; CdSe, In₂Se₃, WSe₂ HgS, PbSe, CdTe and other metalchalcogenides; and GaAs, Si, Se, Cd₂P₃, Zn₂P₃, InP, AgBr, PbI₂, HgI₂,BiI₃ and other semiconductors. Also usable are composites comprising atleast one member selected from the above semiconductors.

Preferable semiconductor particles are titanium oxide particles, whichare inexpensive and highly photocatalytic. Anatase-type titanium oxideparticles are particularly preferable due to its remarkably highphotocatalytic activity.

Titanium oxide particles are commercially available: commercial productsinclude “AMT-600” (tradename of TAYCA, anatase-type, mean primaryparticle size: 30 nm), “AMT-100” (tradename of TAYCA, anatase-type, meanprimary particle size: 6 nm), “ST-01” (tradename of Ishihara Techno,anatase-type, mean primary particle size: 7 nm), “ST-21” (tradename ofIshihara Techno, anatase-type, mean primary particle size: 20 nm),“P-25” (tradename of Nippon Aerosil, rutile-anatase type, mean primaryparticle size: about 30 nm), etc.

The mean primary particle size of the semiconductor particles is, forexample, about 1 nm to about 1000 nm, and preferably about 5 nm to about100 nm. When used for dye-sensitized solar cells (Graetzel cells),semiconductor particles with a mean primary particle size of less than 1nm are not preferable, since the use of such particles leads to asemiconductor layer with a small mean pore size, making it difficult totransfer redox substances in the electrolyte solution and to adsorb thesensitizing dye. As a result, the current value after photoelectricconversion is low. Similarly, semiconductor particles with a meanparticle size greater than 1,000 nm are not preferable since use of suchparticles leads to a semiconductor layer that has too small a surfacearea to support a sufficient amount of sensitizing dye, resulting in alow current value after photoelectric conversion.

The semiconductor particles are preferably dispersed in a solventusually using a dispersing device. Usable dispersing devices includepaint shakers, pebble mills, sand mills, etc. The mean particle size ofthe semiconductor particles after dispersion using the dispersing deviceis preferably about 100 nm or less.

Preferable solvents for dispersing the semiconductor particles includemethanol, ethanol and mixtures thereof. In particular, when usingethanol as the solvent, a dispersion liquid with improved stability canbe obtained due to the excellent affinity of ethanol to semiconductorparticles. Thus, use of ethanol alone or as the main solvent isadvantageous in that the resulting dispersion liquid is unlikely tocohere at the nozzle tip and clog the nozzle, even when the dispersionliquid is subjected to a high shearing force at the time of spraycoating.

When necessary, water and/or an organic solvent can be used incombination with methanol and/or ethanol. Usable organic solventsinclude, for example, xylene, toluene, and other aromatic solvents;n-propanol, isopropanol, n-butanol, polyalkylene glycols and otheralcoholic solvents; diethylene glycol, diethylene glycol monoethylether, diethylene glycol monobutyl ether, triethylene glycol monomethylether, polyoxyalkylene glycol derivatives (e.g.,polyoxyethylene(10)octylphenyl ether) and other ethereal solvents;acetone, methyl ethyl ketone and other ketonic solvents; methyl acetate,ethyl acetate, propyl acetate, butyl acetate, isobutyl acetate and otherester solvents; etc.

The dispersion liquid may contain, in addition to semiconductorparticles, known complexing agents (e.g., acetyl acetone), semiconductorparticle precursors, etc.

The semiconductor particle dispersion liquid for use in the presentinvention preferably has a solids content of about 1 wt. % to about 40wt. %, and preferably about 5 to about 30 wt. %, at the time of spraycoating.

When the dispersion liquid has a solids content less than 1 wt. %, it isdifficult to obtain the necessary film thickness, with the result thatthe obtained semiconductor film is liable to have insufficient energyconversion properties such as photoelectric conversion efficiency.Moreover, with such a solids content, the dispersion liquid as depositedby spray coating is unlikely to have an adequate solids content (usually90 wt. % or more), making it difficult to obtain a thin semiconductorfilm that is porous and has good adhesion.

When the dispersion liquid has a solids content of more than 40 wt. %,the mean diameter of the atomized droplets of the dispersion liquiddischarged from the spray coater is large, making it difficult to obtaina semiconductor film with excellent photoelectric conversion efficiency.Moreover, when the dispersion liquid has a high solids content, themovement of the semiconductor particles in the atomized droplets of thedispersion liquid is restricted, resulting in a reduction ofsemiconductor particle binding energy produced by the sharp decrease ofthe kinetic energy of the semiconductor particles from the start of thespraying of the dispersion liquid until the deposition on the substrate.Thus, it is difficult to obtain a thin semiconductor film with goodadhesion and high strength.

The semiconductor particle dispersion liquid preferably has a viscosityof about 0.001 Pa·sec to about 2 Pa·sec, and more preferably about 0.001Pa·sec to about 1 Pa·sec.

When the viscosity is less than 0.001 Pa·sec, the dispersion liquid isimparted with only a low energy by spraying, and therefore thesemiconductor particles have a low initial energy, making it difficultto obtain a thin semiconductor film with good adhesion. When thedispersion liquid has a viscosity of more than 2 Pa·sec, it is difficultto atomize the dispersion liquid into droplets with a mean diameter ofabout 30 μm or less, resulting in difficulties in forming a thinsemiconductor film with good adhesion.

Spray Coating and Drying of the Coating

Examples of spray coaters usable in the process of the present inventioninclude electrostatic spray coaters, non-electrostatic spray coaters,rotary spray coaters, magnetic spray coaters, ultrasonic atomizers andother known spray coaters. Electrostatic spray coaters and ultrasonicatomizers are particularly preferable.

Various nozzles are usable for such coaters, with two-fluid spraynozzles and other nozzles that can form atomized droplets with a narrowparticle size distribution being preferable.

In the process of the present invention, it is essential that theatomized droplets of the dispersion liquid of semiconductor particlesdischarged from the spray coater have a mean diameter of about 30 μm orless. When the atomized droplets of the dispersion liquid have a meandiameter of more than 30 μm, it is likely that the dispersed particlesclog the nozzle tip or that the atomized droplets of the dispersionliquid deposited on the substrate surface unevenly agglomerate, failingto form a desirable porous film. As a result, it is difficult to obtaina semiconductor film with good adhesion to the substrate and highphotoelectric conversion efficiency. Further, when the atomized dropletsof the dispersion liquid have a large mean diameter, the solventvolatilizes only slowly, and thus the dispersion liquid deposited on thesubstrate is unlikely to have an adequate solids content (usually 90 wt.% or more), and the kinetic energy of the semiconductor particles in theatomized droplets does not sharply decrease. As a result, it isdifficult to obtain a porous film with good adhesion and high strength.

Preferably, the atomized droplets of the dispersion liquid have a meandiameter of about 1 μm to about 25 μm, and more preferably about 1 μm toabout 20 μm.

In this specification and the appended claims, the mean diameter of theatomized droplets of the semiconductor particle dispersion liquiddischarged from the spray coater is measured using a “2600 ParticleSizer” (tradename of Malvern, US).

The mean diameter of the atomized droplets of the semiconductor particledispersion liquid can be controlled to about 30 μm or less by adjustingspraying conditions such as nozzle type, atomization air pressure,pattern width, discharge amount, discharge pressure, coating speed,number of stages (number of coating applications), nozzle-substratedistance, etc. Optimal conditions vary depending on the type of thecoater used. Thus, suitable conditions are selected according to thetype of coater to be used, to form atomized droplets with the abovespecified mean diameter.

Examples of suitable coating conditions are as follows: an atomizationair pressure of 0.5 to 5.0 kgf/cm² and preferably 1.0 to 3.0 kgf/cm²; adischarge amount of 1 to 500 g/min and preferably 10 to 100 g/min; anozzle-substrate distance of 5 to 100 cm and preferably 10 to 50 cm; acoating speed of 1 to 200 m/min and preferably 10 to 100 m/min; a stagenumber of 1 to 100 and preferably 1 to 10.

Since the dispersion liquid of semiconductor particles is applied byspray coating while controlling the atomized droplets of the dispersionliquid to have a mean diameter of 30 μm or less, the process of thepresent invention is capable of forming a porous semiconductor film thathas a larger mean pore size and thus a larger specific surface area thanfilms formed by other coating processes such as roller coating. The meanpore size and specific surface area can be measured by, for example,according to JIS R 1625.

In the process of the present invention, the semiconductor particledispersion liquid is sprayed onto the substrate, and the wet coatinglayer is dried to form a porous semiconductor film.

The wet coating layer of the semiconductor particle dispersion liquidcan be dried by allowing to stand at room temperature, heating,irradiation with electromagnetic waves, or other methods.

Heating can be performed using an electric furnace, gas furnace or likedevice. The heating conditions can be suitably adjusted according to thetype of substrate.

For example, when using an inorganic substrate such as a glass plate,the wet coating layer can be dried and sintered by heating at atemperature of 200° C. or higher, and preferably about 300° C. to about700° C. Suitable heating times vary depending on the heatingtemperature, and are usually about 10 minutes to about 3 hours.

When using a thermoplastic resin substrate such as a high polymer film,to avoid deformation and degradation of the substrate, the wet coatinglayer is preferably heated and dried at a temperature of about 200° C.or lower, and more preferably about 130° C. to about 180° C. Thesuitable heating time is usually about 10 minutes to about 3 hours.

Electromagnetic waves usable for drying are not limited as long as theyare capable of imparting energy to the semiconductor particles, andinclude, for example, ultraviolet rays, visible rays, infrared rays,ultrasonic waves, plasma discharges, corona discharges, microwaves, etc.Preferable electromagnetic waves include ultraviolet rays, visible rays,infrared rays and microwaves.

Ultraviolet rays, visible rays, infrared rays (e.g., far-infrared raysand near-infrared rays) and ultrasonic waves can be obtained from xenonlamps, halogen lamps, tungsten lamps, Nernst lamps, Globar lamps,mercury lamps, fluorescent lamps and other lamps; LEDs; ArF excimerlasers, KrF excimer lasers, XeCl excimer lasers, Nd:YAG lasers and otherlasers; synchrotron radiation; sunlight; etc. Microwaves can be obtainedfrom magnetron devices or the like.

When electromagnetic irradiation is employed for drying, it may beperformed in combination with heating. The semiconductor particle layermay further contain semiconductor particle precursors. Further, toremove unnecessary organic or other matter during drying, the drying maybe performed under reduced pressure, under a blown stream of air,oxygen, nitrogen, an inert gas or other gas, or in an ozone atmosphere,an oxidizing atmosphere, a reducing atmosphere or like atmosphere, etc.

In drying by microwave irradiation, the semiconductor particles can besintered by selectively imparting energy to the semiconductor particlesutilizing dielectric loss. Therefore, microwave drying is moreadvantageous than thermal sintering in an electric furnace or likedevice, in that microwave heating is substantially free from heat lossesby heat transfer to the substrate or thermal degradation of thesubstrate, and can be performed in a short period of time. Thus,microwave drying is suitable when the substrate is made of athermoplastic resin.

In microwave drying and sintering, the dielectric constant variesdepending on the composition (e.g., type of dispersion medium; type,properties, particle size and shape of the semiconductor particles;solids content, etc.) of the semiconductor particle dispersion liquid.Thus, suitable conditions, such as microwave radiation frequency,microwave power, and irradiation time, can be selected according to thecomposition.

Suitable conditions for microwave irradiation are selected according tothe type of the semiconductor particle dispersion liquid, and usually asfollows: a frequency of about 300 MHz to about 300 GHz, preferably about600 MHz to about 200 GHz, and more preferably about 1 GHz to about 100GHz; an output of 0.01 kW to about 10 kW, preferably about 0.1 kW toabout 5 kW, and more preferably about 0.2 kW to about 1.0 kW; and anirradiation time of about 1 second to about 60 minutes, preferably about2 seconds to about 30 minutes, and more preferably about 30 seconds toabout 20 minutes.

Usable microwave devices include, for example, an electromagnetic wavethermal sintering device manufactured by Fujidenpa Kogyo Co., Ltd.(tradename “FMS-10-28”, frequency: 28 GHz, output: 1 to 10 kW).

This microwave device emits microwaves with a frequency of 28 GHz and awavelength of 10.7 cm, which are shorter in wavelength than microwavesgenerated by household microwave ovens (frequency: 2.45 GHz, wavelength:12 cm). Accordingly, the device is advantageous in that it can uniformlyheat the coating layer to form a homogeneous semiconductor film, andthat, even when used at a high output, does not cause sparking at theedge or other portions of the coating. Therefore, the device isespecially suitable for sintering semiconductor particles on the surfaceof a high polymer film with a large area.

When using the microwaves to sinter semiconductor particles applied on ahigh polymer film such as a polyethylene terephthalate film with arelatively low melting point, the sintering temperature is preferably atemperature at which polyethylene terephthalate does not deform ordegrade. An example of such a temperature is 200° C. or less, inparticular about 130° C. to about 180° C.

Further, before microwave sintering, the back of the high polymer film(the side opposite to the side to be provided with the semiconductorfilm) may be provided with a plate with a high thermal conductivity,such as a plate of iron, stainless steel, copper or like metal, or aglass plate or like inorganic radiator plate, to release heat from thehigh polymer film.

When the semiconductor coating formed on the high polymer film surfacehas a large area, microwaves are likely to unevenly sinter thesemiconductor particles. Uniform sintering can be achieved by, forexample, the following methods:

(1) Partial irradiation of the semiconductor coating layer withmicrowaves is repeated several times as required so that finally thewhole surface of the coating is irradiated, to thereby dissipate theheat generated by irradiation.

(2) The high polymer film surface is partially coated with thesemiconductor particle dispersion liquid so as to form a stripedpattern, in order to prevent unnecessary heat generation.

(3) A high polymer film strip coated with the semiconductor particledispersion liquid is moved with the dispersion liquid-coated surfacebeing positioned perpendicularly to the direction of the microwaveirradiation.

When the high polymer film is provided with electrodes, the electrodesare masked with a polyimide film or like highly heat-resistant film, andthe non-masked portion is coated with the semiconductor particledispersion liquid, and then the resulting coating is irradiated withmicrowaves for sintering.

Before partially or wholly irradiating the semiconductor particledispersion liquid layer formed on the high polymer film surface withmicrowaves for sintering, a plate made of glass, tetrafluoroethylene orlike material that transmits microwaves may as required be providedabove the semiconductor particle coating layer, to thereby preventsparks or like problems. Further, to achieve uniform heating, the highpolymer film is preferably pressed against and closely contacted withthe worktable to thereby transfer excess heat generated by the microwavesintering to the worktable.

This drying step forms a porous semiconductor film on the substrate. Thethickness of the porous semiconductor film can be suitably selectedaccording to the intended use, and is usually about 1 μm to about 100μm, and preferably about 2 μm to about 50 μm.

Use of the Semiconductor Film Formed by the Process of the PresentInvention

The porous semiconductor film formed on the substrate by the process ofthe present invention has high adhesion, high strength and otherexcellent properties. When formed using a semiconductor withphotocatalytic properties, such as titanium oxide, the film is useful asa photocatalyst, a photoelectrode for dye-sensitized solar cells, etc.

Photocatalyst

A photocatalyst comprising a porous film of titanium oxide or likesemiconductor formed on a substrate by the process of the presentinvention has excellent properties such as antibacterial properties,hydrophilicity, stain resistance, anti-fogging properties, gasdecomposing properties, deodorizing properties, water-treatingcapabilities, energy conversion properties, etc. The poroussemiconductor film is preferably a porous titanium oxide film, and morepreferably a porous anatase-type titanium oxide film, in view of itsexcellent photocatalytic activity.

The photocatalyst of the present invention can be suitably used in thefields of, for example, atmospheric purification, water purification,hydrophilization, antibacterial treatment, deodorization treatment,anti-fogging treatment, wastewater treatment, energy conversion, etc.

Photoelectrode for Dye-Sensitized Solar Cells

The photocatalyst of the present invention can be suitably used in thefield of energy conversion, and in particular, as a photoelectrode fordye-sensitized solar cells.

That is, the present invention also provides a photoelectrode fordye-sensitized solar cells that comprises a porous semiconductor filmformed by the process of the present invention on an electricallyconductive transparent layer provided on a glass plate or a transparenthigh polymer film. The photoelectrode is a laminate of a glass plate ora transparent high polymer film, an electrically conductive transparentlayer, and a porous semiconductor film, superposed in that order.

From the viewpoint of photoelectrode performance, the poroussemiconductor film is preferably a porous titanium oxide film, and morepreferably a porous anatase-type titanium oxide film.

Generally, a dye-sensitized solar cell comprises a photoelectrodecomprising a transparent substrate (e.g., a glass plate or a highpolymer film) and, on one side of the substrate, an electricallyconductive transparent layer and a semiconductor layer; a counterelectrode facing the semiconductor layer; and an electrolyte sandwichedbetween the electrodes.

The semiconductor layer is usually a porous film that is made ofsemiconductor particles and has a large mean pore size, and supports aphotosensitizing dye on the particle surfaces or in the pores.

The electrically conductive transparent layer is formed on the surfaceof a glass plate, a high polymer film or like transparent substrate byvapor-depositing gold, silver, aluminum, indium, indium tin oxide (ITO),tin oxide or the like, and usually has a thickness of about 0.01 μm toabout 500 μm, and preferably about 0.1 μm to about 100 μm.

The porous semiconductor film formed by the process of the presentinvention is used as the semiconductor layer. The porous semiconductorfilm is usually about 1 μm to about 100 μm thick, and preferably about 2μm to about 50 μm thick.

The photosensitizing dye can be selected from various known dyes thatabsorb light in the visible region and/or the infrared region of thespectrum.

Examples of photosensitizing dyes include azo dyes, quinone dyes,quinoneimine dyes, quinacridone dyes, squarylium dyes, cyanine dyes,merocyanine dyes, triphenylmethane dyes, xanthene dyes, porphyrin dyes,phthalocyanine dyes, perylene dyes, indigo dyes, naphthalocyanine dyes,etc. Among these, phthalocyanine dyes and naphthalocyanine dyes have ahigh quantum yield and high durability against light, and therefore arepreferable as photoelectric conversion materials.

Examples of metals to be combined with the photosensitizing dye includecopper, nickel, iron, cobalt, vanadium, tin, silicon, titanium,germanium, cobalt, zinc, ruthenium, magnesium, aluminum, lead,manganese, indium, molybdenum, zirconium, antimony, tungsten, platinum,bismuth, selenium, silver, cadmium, platinum, etc. Among these, copper,titanium, zinc, aluminum, iron, vanadium and silicon are preferablesince metal complex dyes comprising these metals have a high quantumefficiency.

The amount of the photosensitizing dye to be supported by thesemiconductor particles is preferably about 10⁻⁸ mol/cm² to about 10⁻⁶mol/cm², and more preferably about 0.1 to 9.0×10⁻⁷ mol/cm². Less than10⁻⁸ mol/cm² of photosensitizing dye does not sufficiently improve thephotoelectric conversion efficiency. More than 10⁻⁶ mol/cm² ofphotosensitizing dye does not further improve the photoelectricconversion efficiency, and thus is uneconomical.

The electrolyte used in the electrolyte layer is not limited as long asit comprises a redox pair in a solvent. Preferably, the redox pairconsists of an oxidant and reductant with the same electric charge. Theredox pair is a pair of substances that reversibly exist in an oxidizedor reduced form in an oxidation-reduction reaction system. Redox pairsare well known to persons of ordinary skill in the art.

Examples of redox pairs include chlorine compound-chlorine, iodinecompound-iodine, bromine compound-bromine, thallium ion (III)-thalliumion (I), mercury ion (II)-mercury ion (I), ruthenium ion (III)-rutheniumion (II), copper ion (II)-copper ion (I), iron ion (III)-iron ion (II),vanadium ion (III)-vanadium ion (II), manganic acid ion-permanganic acidion, ferricyanide-ferrocyanide, quinone-hydroquinone, fumaricacid-succinic acid, etc. Other redox pairs are also usable.

Among the above redox pairs, iodine compound-iodine is preferable.Preferable iodine compounds include lithium iodide, potassium iodide,copper iodide, silver rubidium iodide and other metal iodides;tetraalkylammonium iodine, pyridinium iodine and other quaternaryammonium iodide salt compounds; dimethylpropylimidazolium iodide andother diimidazolium iodide compounds; etc.

The solvent to be used for dissolving the electrolyte is preferably acompound that dissolves the redox pair and has high ionic conductivity.Water and/or an organic solvent can be used as a solvent. Preferably, anorganic solvent is used in order to stabilize the redox pair.

Specific examples of organic solvents include dimethyl carbonate,diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, propylenecarbonate and other carbonate compounds; methyl acetate, methylpropionate, γ-butyrolactone and other ester compounds; diethyl ether,1,2-dimethoxyethane, 1,3-dioxolane, tetrahydrofuran,2-methyltetrahydrofuran and other ether compounds;3-methyl-2-oxazolidine, 2-methylpyrrolidone and other heterocycliccompounds; acetonitrile, methoxyacetonitrile, propionitrile and othernitrile compounds; sulfolane, dimethylsulfoxide, dimethylformamide andother aprotic polar compounds; etc. Such solvents may be used singly orin combination. Especially preferable solvents include ethylenecarbonate, propylene carbonate and other carbonate compounds;3-methyl-2-oxazolidine, 2-methylpyrrolidone and other heterocycliccompounds; and acetonitrile, methoxyacetonitrile, propionitrile andother nitrile compounds.

The electrolyte may be in the form of a liquid, solid or gel.

An adhesive layer may be provided outside the counter electrode of thedye-sensitized solar cell comprising a photoelectrode that comprises atransparent substrate (e.g., a glass plate or high polymer film)provided on one side with an electrically conductive transparent layerand semiconductor layer; an electrolyte; and a counter electrode facingthe semiconductor layer.

An adhesive layer makes the solar cell attachable to various articles.

The solar cell can be preferably attached to, for example, vehicles,buildings, constructions, roads, traffic signs, greenhouses and otherstructures that are likely to be exposed to sunlight.

The solar cell may be attached to a plastic plate, metal plate or likeitem by bringing the surface of the adhesive layer into contact with theitem, followed by compression optionally with heating, to thereby bondthe solar cell to the item. An item with a solar cell bonded thereto canbe formed into a module by molding, cutting and/or other processes asrequired, followed where necessary by sealing of the cut portions.

When the solar cell is intended to be cut, a solid or gel electrolyte isused therein.

Examples of adhesives usable in the adhesive layer include knownpressure-sensitive adhesives, heat-sensitive adhesives, curableadhesives, etc. Specific examples include thermosetting andthermoplastic adhesives comprising at least one member selected from thegroup consisting of bisphenol-type epoxy resins, resol-type epoxyresins, acrylic resins, aminoplast resins, polyester resins, urethaneresins, polysiloxane resins, butylene resins, isobutylene resins, vinylacetate resins, vinyl chloride resins, vinyl chloride/vinyl acetatecopolymers, synthetic rubbers, natural rubbers, etc.

The adhesive layer is preferably 1 μm to 1 mm thick, and more preferably5 μm to 500 mm thick.

The process of the present invention, in which a semiconductor particledispersion liquid is applied to a substrate by spray coating in such amanner that the atomized droplets of the dispersion liquid have a meandiameter of about 30 μm or less, has the following advantages:

(1) After application of the dispersion liquid, semiconductor particlesor aggregates thereof remain on the substrate and form a porous filmthat has excellent adhesion to the substrate and high strength.

Such a porous film can be formed presumably because the semiconductorparticles have an increased binding energy due to the rapid decrease oftheir kinetic energy from the start of the spraying to the deposition tothe substrate.

(2) When the solvent of the semiconductor particle dispersion liquidconsists only or mainly of ethanol, the dispersion liquid is stablebecause of the high affinity of ethanol to the semiconductor particles.Therefore, the dispersion liquid, even when being subjected to a highshear stress at the time of spray coating, is unlikely to cohere at thenozzle tip and thereby clog the nozzle.

Further, the high affinity of ethanol facilitates necking of the coatingfilm formed from the semiconductor particle dispersion liquid, and thuseven when the dispersion liquid is applied to a flexible substrate suchas a high polymer film, it does not peel off when the substrate flexes.

(3) Since the solvent partially evaporates from the dispersion liquidduring spray coating, the semiconductor particles being sprayed arestabilized.

(4) In prior art techniques, a semiconductor particle dispersion liquidis applied to an inorganic substrate such as a glass plate, and sinteredat 200° C. or higher to form a porous semiconductor film. In contrast,the process of the present invention is capable of forming a poroussemiconductor film at low temperatures not higher than 200° C., andtherefore can be carried out using a thermoplastic resin substrate suchas a high polymer film.

(5) By microwave sintering of the semiconductor particle dispersionliquid applied to a high polymer film substrate such as a polyethyleneterephthalate, a porous film can be formed in which the semiconductorparticles are uniformly sintered, even when the film has a large area.

(6) The process is capable of forming, on a substrate, a porous film ofa semiconductor with photocatalytic activities, such as titanium oxide,to obtain a photocatalyst or photoelectrode for dye-sensitized solarcells with excellent performance.

BEST MODE FOR CARRYING OUT THE INVENTION

The following Production Examples, Examples and Comparative Examples aregiven to illustrate the present invention in further detail, and are notintended to limit the scope of the invention. In these examples, partsand percentages are all by weight.

Production Example 1

Thirty parts of “P-25” (tradename of Japan Aerosil Co., Ltd.,rutile-anatase-type titanium oxide (TiO₂) crystals, mean primaryparticle size: 30 nm) and 120 parts of ethanol were agitated with glassbeads in a paint shaker for 6 hours, to thereby obtain a dispersionliquid of titanium oxide particles. The dispersion liquid had aviscosity of 1 Pa·sec.

Example 1

(i) A polyethylene terephthalate (PET) film (100 cm long, 30 cm wide and1 mm thick) was spray-coated with the above titanium oxide particledispersion liquid using a spray coater in which the dispersion liquidwas pumped through a two-fluid spray nozzle (tradename “Atomax Nozzle(Model AM25S)”, manufactured by Atomax Co., Ltd.), under the coatingconditions presented in Table 1. The mean diameter of the atomizeddroplets of the dispersion liquid discharged from the spray coater was19.7 μm.

The resulting coating was dried in an electric furnace at 150° C. for 30minutes, to thereby obtain a 8 μm thick porous titanium oxide film.

The mean pore size of the porous titanium oxide film was measured by themethod described below, and found to be 14.9 nm, demonstrating that apreferable porous film with a large pore size was formed.

Method of measuring the mean pore size: The PET film with the 8 μm thickporous titanium oxide film obtained above was cut into strips (5 mm×25mm), and sixty of the strips were inserted into a glass cell of“MICROMERITICS ASAP2010” (tradename of Shimadzu Corp., pore sizedistribution measuring apparatus utilizing nitrogenadsorption/desorption) to measure the mean pore size.

(ii) An 8 μm thick porous titanium oxide film was obtained in the samemanner as in (i) except using as a substrate an electrically conductivePET film on which a 1 μm thick ITO electrically conductive transparentlayer had been vapor-deposited.

The substrate with the porous film was immersed at room temperatureovernight in an ethanol solution containing a sensitizing dye[Ru(2,2′-bipyridil-4,4′-dicarboxylate(TBA))₂(NCS)] at a concentration of5×10⁻⁴M/L to obtain a photoelectrode. The above-mentioned electricallyconductive PET film sputtered with platinum was superimposed as thecounter electrode on the sensitizing dye-adsorbing porous titanium oxidefilm of the photoelectrode.

An electrolyte solution (40 mM iodine, 500 mM tetrapropylammoniumiodine, 80 M ethylene carbonate and 20 M acetonitrile) was pouredbetween the titanium oxide film of the photoelectrode and the counterelectrode, to obtain a dye-sensitized solar cell.

Example 2

(i) Spray coating was performed in the same manner as in Example 1 (i)except for employing the coating conditions shown in Table 1. The meandiameter of the atomized droplets of the dispersion liquid dischargedfrom the spray coater was 24.3 μm.

The resulting coating was dried in an electric furnace at 150° C. for 30minutes, to thereby obtain a 9 μm thick porous titanium oxide film.

The mean pore size of the porous titanium oxide film was measured in thesame manner as in Example 1 (i) and found to be 14.5 nm, demonstratingthat a preferable porous film with a large pore size was formed.

(ii) A dye-sensitized solar cell was obtained in the same manner as inExample 1 (ii) except for employing the coating conditions shown inTable 1 and forming a 9 μm thick porous titanium oxide film.

Example 3

(i) Spray coating was performed in the same manner as in Example 1 (i)except for employing the coating conditions shown in Table 1. The meandiameter of the atomized droplets of the dispersion liquid dischargedfrom the spray coater was 19.5 μm.

The resulting coating was irradiated with microwaves using anelectromagnetic wave thermal sintering apparatus (tradename “FMS-10-28”,manufactured by Fujidenpa Kogyo Co., Ltd.) at a frequency of 28 GHz, anoutput of 2 kW and an irradiation time of 2 minutes, to thereby obtain a9 μm thick sintered porous titanium oxide film.

The mean pore size of the porous titanium oxide film was measured in thesame manner as in Example 1 (i) and found to be 15.5 nm, demonstratingthat a preferable porous film with a large pore size was formed.

(ii) Spray coating was performed under the coating conditions shown inTable 1 and the coating was irradiated with microwaves in the samemanner as above, to thereby obtain a 9 μM thick porous titanium oxidefilm. Using the obtained substrate with the film, a dye-sensitized solarcell was produced in the same manner as in Example 1 (ii).

Comparative Example 1

The titanium oxide particle dispersion liquid obtained in ProductionExample 1 was applied with a knife coater to a PET film (100 cm long, 30cm wide and 1 mm thick).

The resulting coating was dried in an electric furnace at 150° C. for 30minutes, to thereby obtain a 8 μm thick porous titanium oxide film.

The mean pore size of the porous titanium oxide film was measured in thesame manner as in Example 1 (i) and found to be 9.5 nm, demonstratingthat an inferior porous film with a small pore size was formed.

Comparative Example 2

Spray coating was carried out in the same manner as in Example 1 (i)except for employing the coating conditions shown in Table 1. The meandiameter of the atomized droplets of the dispersion liquid dischargedfrom the spray coater was 41.4 μm.

The resulting coating was dried in an electric furnace at 150° C. for 30minutes, to thereby obtain a 10 μm thick porous titanium oxide film.

The mean pore size of the porous titanium oxide film was measured in thesame manner as in Example 1 (i) and found to be 10.5 nm, demonstratingthat an inferior porous film with a small mean pore size was formed.

Table 1 presents the spray coating conditions employed in Examples 1 to3 and Comparative Example 2. TABLE 1 Ex. 1 Ex. 2 Ex. 3 Comp. Ex. 2Discharged amount (g/min) 60 60 60 60 Atomization air pressure 3.0 2.02.0 1.0 (kgf/cm²) Number of stages 3 3 3 3 Nozzle-substrate 20 20 20 20distance (mm) Coating rate (m/min) 60 60 60 60

The porous titanium oxide films obtained in Example 1 (i), Example 2(i), Example 3 (i), and Comparative Examples 1 and 2 were tested foradhesion and scratch resistance. Further, the photoelectric conversionefficiency of the dye-sensitized solar cells obtained in Example 1 (ii),Example 2 (ii) and Example 3 (ii) was measured. The test methods are asfollows.

Adhesion: The adhesion of the porous titanium oxide films to the PETfilm was tested by a bending test. A porous film that did not peel offeven when the PET film with the porous film formed thereon was sharplybent or forcefully hit was evaluated as having good adhesion, while aporous film that peeled off when the PET film with the porous film wasbent was evaluated as having poor adhesion.

Film scratch resistance (gf): Porous titanium oxide films were formed inthe same manner as in the Examples and Comparative Examples except forusing, as the substrate, a glass plate in place of the PET film, andused as test samples.

Using a “Tribogear Type 18L” (tradename of Shinto Scientific Co., Ltd.,continuous load scratch resistance tester), the load at which the glassplate was uncovered was found by applying a vertical load of 0 to 100 gto a scratch needle (made of sapphire, 1.2 mm in diameter) and movingthe needle at a rate of 600 mm/min over a distance of 100 mm. Thegreater the load, the higher the film strength. A load of 10 gf or moreis particularly preferable.

Photoelectric conversion efficiency (%): The dye-sensitized solar cellswere irradiated with artificial sunlight (xenon lamp) (AM1.5, unit: 100mW/cm²) to measure the photoelectric conversion efficiency.

Table 2 shows the test results. TABLE 2 Ex. 1 Ex. 2 Ex. 3 Comp. Ex. 1Comp. Ex. 2 Adhesion Good Good Good Poor Poor Scratch 14.1 12.6 12.6 6.17.3 resistance Photoelectric 2.1 1.6 2.8 — — conversion efficiency

1. A process for forming a semiconductor film, comprising the steps of:applying a semiconductor particle dispersion liquid to a substratesurface by spray coating in such a manner that the atomized droplets ofthe dispersion liquid discharged from the spray coater have a meandiameter of 30 μm or less; and drying the coating to form a poroussemiconductor film.
 2. The process according to claim 1, wherein thesubstrate is a thermoplastic resin substrate.
 3. The process accordingto claim 2, wherein the thermoplastic resin substrate is a high polymerfilm.
 4. The process according to claim 1, wherein the semiconductorparticle dispersion liquid is a dispersion in methanol and/or ethanol ofparticles of at least one semiconductor selected from the groupconsisting of metal oxides, perovskites, metal sulfides and metalchalcogenides.
 5. The process according to claim 4, wherein thesemiconductor particles are titanium oxide particles.
 6. The processaccording to claim 5, wherein the titanium oxide particles areanatase-type titanium oxide particles.
 7. The process according to claim1, wherein the semiconductor particle dispersion liquid has a solidscontent of about 1 wt. % to about 40 wt. %.
 8. The process according toclaim 1, wherein the semiconductor particle dispersion liquid has aviscosity of about 0.001 Pa·sec to about 2 Pa·sec.
 9. The processaccording to claim 1, wherein the atomized droplets of the dispersionliquid discharged from the spray coater have a mean diameter of about 1μm to about 25 μm.
 10. The process according to claim 1, wherein thecoating is dried by heating at a temperature of about 200° C. or loweror by irradiation with electromagnetic waves.
 11. The process accordingto claim 10, wherein the coating is dried by microwave irradiation. 12.A photocatalyst comprising a porous semiconductor film formed on asubstrate by the process according to claim
 1. 13. The photocatalystaccording to claim 12, wherein the porous semiconductor film is a poroustitanium oxide film.
 14. The photocatalyst according to claim 13,wherein the porous titanium oxide film is a porous anatase-type titaniumoxide film.
 15. A photoelectrode for dye-sensitized solar cells,comprising a porous semiconductor film formed by the process accordingto claim 1 on an electrically conductive transparent layer formed oneither a glass plate or a transparent high polymer film.
 16. Thephotoelectrode according to claim 15, wherein the porous semiconductorfilm is a porous titanium oxide film.
 17. The photoelectrode accordingto claim 16, wherein the porous titanium oxide film is a porousanatase-type titanium oxide film.